The birth of a human child is a complicated and traumatic process, of whose mechanical and geometrical aspects, surprisingly little is known. In a normal birth, the cervix dilates and a fetus is pushed out from a uterus and along a birth canal (e.g., the vagina), which lies between the pubic bones. This is described in greater detail, for example, in Williams Obstetrics Cunningham, Gary et al. Appleton & Lange 20th edition, the disclosure of which is incorporated herein by reference. Due to the relatively large size of the head of the fetus, not only is the head distorted by this passage, but it must also change direction and turn during the passage. First, however, the head must exit the uterus through the cervix, which requires the cervix to dilate a large amount.
It is common practice to monitor the progress of birth by measuring the degree of cervical dilation and monitoring contraction frequency (other parameters may be monitored as well, such as maternal heart rate). At a later stage, birth progress is estimated based on the location of the fetus's head. The locations along the birth canal are defined as stations. Currently, the gold standard of measurement for dilation, head orientation, station, effacement and cervical consistency is the human hand. However, this measurement method is not only unsanitary, it is also intrusive and inaccurate. Since the accuracy and meaning of the measurements depends on the person measuring (and even for a same person a 1 cm error is considered normal), when shifts change, measurements change. In addition, it is hypothesized that the cervix dilation increases momentarily during contraction, and fetus head location and orientation changes. These variations cannot be measured continuously in a reliable manner manually, reducing the amount of information available, for example, the effect of each contraction.
Various mechanical and electrical systems have been devised to measure cervical dilation and/or fetus head location, for example as described in “Cervimetry: A Review of Methods for Measuring Cervical Dilation During Labor”, Obstetrics & Gynecology Survey, Vol. 55, No. 5, 2000 and U.S. Pat. Nos. 5,222,485, 6,039,701, 6,246,898, 5,935,061 and 6,200,279, the disclosures of which are incorporated herein by reference. These methods include, for example, Obstetric gloves incorporating a measuring string, finger mounted angular V calipers, Cervix mounted angular V calipers, induction transmitters and receivers clamped to two sides of the cervix opposite each other, a multi-switch membrane inserted in the uterus and pressed between the cervical internal os and the fetal head and magnetic filed position sensors.
A Ph.D. thesis By Robert Neal Wolfson, of September 1974, submitted to Case Western University, Department of Biomedical Engineering and titled “An Instrument for the Continuous and Quantitative Determination of Fetal Descent by Measurement of Ultrasonic Transit Time (1975)”, the disclosure of which is incorporated herein by reference, suggests using ultrasonic waves and triangulation to determine a location of a fetus's head, using ultrasonic sensors placed inside the body to detect an ultrasonic field generated outside the body. One potential problem with this approach is that wires are required to connect to the ultrasonic sensors inside the body. Providing ultrasonic transmitters inside the body may not be a viable solution, due to their size. Using ultrasonic transmitter/receivers or reflectors may also be difficult, since they require transmission of ultrasound through tissue which may suffer from noise degradation and other effects (e.g., due to the many interfaces between different materials, significant accuracy and signal to noise problems may be experienced).